1. Field of the Invention
This invention relates generally to scanning force microscopes, and more particularly concerns a scanning atomic force microscope with an oscillating cantilever probe and a modulated resonance contact mode of operating the microscope for imaging surface contours of a specimen.
2. Description of Related Art
Scanning force microscopes, also known as atomic force microscopes, are useful for imaging objects as small as atoms. Scanning force microscopy is closely related to scanning tunneling microscopy and the technique of stylus profilometry. In a scanning force microscope, a laser beam is typically deflected by the free end of a reflective lever arm to which the probe is mounted, indicative of vertical movement of the probe as it follows the contours of a specimen. The deflection of the laser beam is typically monitored by a photosensor in the optical path of the deflected laser beam, and the sample is mounted on a stage moveable in minute distances in three dimensions. The sample can be raster scanned while the vertical positioning of the probe relative to the surface of the sample is maintained substantially constant by a feedback loop with the photosensor controlling the vertical positioning of the sample.
The interactive forces between the probe and surface of the specimen change at different distances. As the probe approaches the surface of an uncontaminated specimen, it is initially attracted to the surface by relatively long range attractive forces, such as van der Waals forces. As the probe tip approaches further, repulsive forces from the electron orbitals of the atoms on the probe tip and the specimen surface become more significant. Under normal ambient conditions, the surface of a specimen will also be covered by a thin contamination layer, typically composed of water and other ambient contaminants, and contaminants remaining from production of the specimen. The thickness of the contamination layer can vary due to humidity and specific ambient conditions, but is generally between 25 and 500 .ANG.. This contamination layer can also have an interactive effect on the probe tip. As the probe tip approaches the contamination layer of a specimen, capillary surface forces can strongly attract the probe tip toward the surface of the specimen. When the probe tip is being retracted from the surface of the specimen, the capillary attraction forces can also strongly resist retraction of the probe tip from the surface of the specimen.
In conventional non-modulated modes of operating atomic force microscopes, where the lever arm is not oscillated, output from the detector monitoring the deflection of the reflective probe lever arm is typically used as feedback to adjust the position of the probe tip to maintain the interactive forces and distance between the probe tip and specimen surface substantially constant. In a conventional non-modulated, DC-contact mode of operation, the detected displacement of the probe is used in a feedback loop to adjust the position of the probe so that the force between the probe and the specimen surface remains substantially constant. It has been observed that in a non-modulated contact scanning mode, high rates of scanning, i.e. at four scan lines per second over a 50 micron range, can result in a hydroplaning effect, with the probe tip skimming over the surface of a contaminant layer, causing an unusual amount of noise to be present in the output signal.
In modulated modes of operating a scanning force microscope, the reflective lever arm is typically mounted to a piezoelectric ceramic material which can be driven by an alternating voltage to cause the lever arm and the probe tip to oscillate at a desired frequency. In modulated "non-contact" and "intermittent contact" scanning modes, as the oscillating probe tip approaches the surface of the specimen, both the amplitude and phase shift of the probe relative to the driving oscillator are perturbed by the surface forces. Measurements are typically made of the average cantilever amplitude or the shift in phase of the cantilever relative to the driven oscillation, in order to monitor the interaction of the tip with the attractive and repulsive forces of the surface of the sample, generally due to a contaminant layer on the surface of the sample, in ambient, open air conditions. Either the change in amplitude or the change in phase can typically be used in a positioning feedback loop.
In a conventional high amplitude resonance modulation mode, in which the probe is oscillated at its resonant frequency, typically at 50-500 kHz, at a high amplitude of from 100 to 1,000 .ANG., the probe has intermittent contact with the surface of the specimen, rapidly moving in and out of the contamination layer. In this mode, the topographical image is not significantly affected by the contamination layer, since the probe rapidly penetrates this layer. Either the probe or the sample can be damaged in this mode, which is more appropriate for imaging soft specimens. In a conventional low amplitude resonance mode, in which the probe tip is also typically oscillated at it resonance frequency at from 50-500 KHz at a low amplitude, the probe remains within the contamination layer, in the attractive region. However, since the contamination layer can change, due to warming of the specimen, changes in humidity or other ambient surface conditions, images made with in this mode of operation can also change unpredictably.
Resonance modes of operation also present special problems, in that changes in amplitude and phase during oscillation of the lever arm due to long and short range forces occurring between the tip and the surface of the sample are most greatly affected when the frequency is at or near the fundamental resonance frequency. At resonance, the oscillation is quickly damped when the probe tip is at or near the sample surface. The quality factor, Q, of the oscillating lever arm at resonance further increases the effect of the interacting surface forces on the amplitude and phase shift. For a single optical lever arm made of silicon (100 microns long, 15 microns wide, 6 microns thick), the resonance frequency is about 300 Khz, and the Q factor is well over 100 in air. However, operation of a scanning force microscope with a lever arm having a high Q factor in "non-contact" mode at the resonance frequency can cause "ringing" problems, reducing frequency response. Consequently, conventional resonance modes of operation typically result in low resolution imaging of the surface of a specimen.
It has been found that the quality of scanning force microscope images for small scan ranges, i.e. less than about 1 micron, and with surfaces having small features, i.e. less than about 20 nm, is limited by noise from the dynamics of tip-surface interaction and acoustic noise. Acoustic noise combined with possible resonance feedback from a normal laboratory environment can also result in reduced image resolution, particularly when a probe is oscillated at lower frequencies, such as from 10-100 KHz. At high scan rates, such as five scan lines per second over a 400 nm range, acoustic noise can be as much as 10 .ANG. in comparison to features of from 10-15 .ANG.. Factors such as feedback control settings, the scan rate, and the frequency characteristics of the lever arm of the probe can affect the amount of noise encountered, but tip-surface interaction noise and acoustic noise typically can easily exceed design performance of the microscope, causing streaks in the images of specimens which are much longer than the effective tip-contact radius.
It would be desirable to provide a way of overcoming problems of noise and differences in interaction of the probe tip with a contamination layer to provide for high resolution imaging at high scan rates. The present invention addresses these needs.